The present invention relates in general to amplifier circuits and, more particularly, to a variable gain amplifier circuit.
Variable gain amplifier circuits are used in a myriad of applications, including receivers, transmitters and high fidelity electronics, anywhere it is necessary to adjust the amplitude of a signal. Specific circuits include attenuators, volume controls, tone controls, audio mixers, and companders.
A typical gain amplifier circuit is disclosed in FIG. 1 of U.S. Pat. No. 4,878,031 and includes an input stage and an output stage connected through a common node to the output of an op amp which in turn controls the current in each of the stages. The op amp senses a particular voltage, set by the magnitude of input current, and adjusts its output voltage to compel the input stage to sink the input current. The same op amp output voltage is applied to the output stage and controls the magnitude of the output current. Thus, a particular input current results in an particular output current proportional to the ratio of the current sources within the respective input and output stages. The input and output terminals receive current sources within the respective stages in such manner that the magnitude of the input and output currents is limited to that of the current source. Equivalently, these current sources must be sufficiently large as to handle the specifications of the input and output signals and, therefore are generally unacceptable in battery applications where power consumption is an issue. Also, in high fidelity and high power applications the excessive current in the variable gain amplifier circuit generates noise through common impedance to other components degrading the performance.
U.S. Pat. No. 4,878,031 further discloses another variable gain amplifier circuit in FIG. 2 that includes an input stage that is responsive to an applied input signal for providing an output signal at an output stage. The input stage and output stage are independently biased by respective bias sources and each include circuitry responsive to a dynamic control voltage, the latter of which is generated in response to the input signal, to permit the absolute magnitudes of the input signal and output signal to exceed the respective bias sources. The ratio of the output and input signals is proportional to the ratio of the bias sources. The circuit arrangement of the U.S. Pat. No. 4,878,031 in FIG. 2 requires an operating potential of say 2.3 volts for proper biasing and headroom. For battery applications it is desirable to further reduce the required operating potential.
Hence, a need exists for a variable gain amplifier circuit requiring low operating potential.